Later that night, a half-cut Dylan and Caitlin are relaxing together in the guest bedroom when Caitlin's sister Nicolette storms in. She drags Caitlin downstairs to show her evidence of the poet having treated an invaluable family heirloom as a urinal.

Fast exit the Thomases. Unrepentant, they care not a fig about their callous abuse of their relatives' hospitality.

The scene is one of many memorable moments in The Edge Of Love, starring Keira Knightley and Sienna Miller.

The film, which opened in British cinemas this weekend, explores the uncomfortable love triangle between the Welsh poet, Caitlin (played by Sienna Miller) and his childhood sweetheart Vera Phillips (Keira Knightley).

For me, it has a particular resonance. I am Nicolette and Anthony's grandson. Dylan and Caitlin Thomas were my great uncle and aunt.

In her autobiography, Two Flamboyant Fathers, Nicolette, a renowned writer, vividly describes the bohemian milieu in which she, her sister Caitlin and their respective families played out their lives.

A Who's Who of Bloomsbury, the book is rich in illuminating insights and anecdotes. Anthony himself was a highly celebrated artist who once painted the Queen.

Nicolette held back from revealing the full extent of Dylan and Caitlin's numerous excesses.

Nonetheless, before the book was published, she wisely took out libel insurance against a prospective defamation writ from her famously vengeful sister.

In Nicolette's eyes, Caitlin was a fascinating force of Nature but something of a monster. Dylan was a genius, she said, but in conduct hardly better than his wife.

Compared to the feelings shared by her son Prosper and daughter Emma, my mother, this assessment was generous.

Nicolette loved confiding in me all the horror stories about my famous great uncle and aunt. Dylan and Caitlin frequently turned up unannounced at her and Anthony's terrace house in Markham Square, Chelsea.

On one occasion, my grandparents asked after the couple's small children. Why weren't they with them? Where were they?

They were being looked after at their home in Wales, said Caitlin.

Not true. The children had simply been abandoned. Fortunately, they were found by accident by Dylan's mother who happened to be passing the house.

As soon as they had invited themselves to my grandparents' London home - a frequent occurrence - Dylan and Caitlin would drink their way round their familiar haunts: the Pheasantry club, the Cooper's Arms, the Queen's Elm and most of the Fitzrovia area.

Caitlin could be cruel as well as shameless. After the Second World War, severe shortages meant rationing. Many children grew up knowing of oranges and bananas only through folklore and pictures.

However, one day my grandfather managed to acquire an orange which he intended his three children to share. It was placed as the centrepiece on the dining table and the excited children were told they'd be tasting the exotic fruit later that evening.

Unfortunately, that afternoon Aunt Caitlin decided to pay one of her many unscheduled visits.

Spying the fruit, she peeled it and ate it - an outrage surpassed only by the writer Evelyn Waugh who, in this same period, called in his children so they could watch him eat the bananas meant only for them.

In 1958 Anthony, my grandfather, died at 47 of a stroke. Nine years later, Nicolette married the artist Rupert Shephard.

Soon after, Caitlin dropped in to see her sister. By now, Nicolette had deduced that alcohol triggered the unleashing of Caitlin's inner monster.

So she took the precaution of locking the drinks cabinet. Decorously, the sisters chatted over a cup of Lapsang Souchong.

Working upstairs, Rupert suddenly heard a commotion. What was going on?

Arriving downstairs, he found his wife semi-conscious. Bravely, Nicolette had tried to stand between Caitlin and the object of her desire - so her sister had punched her to the ground.

While a startled Nicolette nursed her jaw, Caitlin was frantically trying to prise open the cabinet.

Sienna Miller, right, as Caitlin and Keira Knightley as Vera in The Edge Of Love

This time, though, she had more than met her match. Rupert, a big bear of a man, promptly ejected Caitlin from the house.

How I would have loved to have taken Nicolette to see The Edge Of Love. I can imagine her now, her undaunted face a conflicting compound of scorn, pride and mirth at the depiction of her family.

What would have tickled her most, I suspect, was Sienna speaking in a distinct Irish brogue. Caitlin was a sprig of Anglo-Irish gentry. But for the occasional Irish inflection, her accent was almost BBC.

The film received early notoriety when rumours abounded that the script contained lesbian love scenes between Caitlin and other women. The finished film has no explicit moments between Caitlin and Vera Phillips, but lesbianism is implied.

Why such primness? Caitlin herself was remarkably free of such bourgeois restraints. After all, when Caitlin and Nicolette's father walked out on the family, they were taken in by the notoriously promiscuous artist Augustus John. He became the children's unofficial stepfather - the other 'flamboyant father' in my grandmother's memoir - and had an affair with Caitlin when she was just 15.

From then on, Caitlin rarely seemed to pass up any sexual opportunity. Apparently she once had sex with 12 Irish labourers, one after the other, in the back room of a pub.

Caitlin's appetite was by no means restricted to men. Legend has it that on her wedding night she took one of her bridesmaids into her marital bed.

No evidence exists of Caitlin being amorous with Dylan's childhood love Vera Phillips, but from what I know about her it is distinctly possible.

While I was in my early 20s, Nicolette and I became very close. It was not just that I was the first of her grandchildren, I had become a writer, her chosen profession.

I had worked for ten weeks with Sir John Betjeman, a sound apprenticeship; had a long poem published in a Sunday broadsheet and was selling articles to magazines and newspapers. She took an avid interest in my development.

One afternoon I dropped round for tea. She told me she had heard I was heading off to Italy for a holiday.

She then pressed into my palm a scrap of paper with a number and name. The number was a telephone in Rome. The name was Caitlin.

After Dylan's death in 1953, she had moved to Italy where she had married Giuseppe Fazio, a fiery Sicilian.

Nicolette told me it was imperative I met up with Caitlin and got to know her. After all, she insisted, I was the next generation of writer and the family link must be maintained.

At that point Dylan and Caitlin's daughter Aeronwy, who now carries the family torch, had yet to show her writing promise.

I tried to explain to Nicolette that I was not visiting Rome, but an island off Naples. But she was not to be deterred by a footling matter of geography. Make contact, Nicolette commanded.

In late Summer 1980 I flew to Naples. On the 40-minute ride to the ferry to Ischia, I found myself fingering the scrap of paper from my grandmother.

I had taken one of five villas owned by the composer Sir William Walton. With me was my girlfriend, Hilary, who the following year was to become my wife.

Joining us later was the novelist Jane Gaskell and her boyfriend. On arriving, Hilary espied in a neighbouring villa the distinctive and statuesque figure of media legend Janet Street-Porter and her then husband.

Thoughts of Caitlin, with whom my grandmother had instructed me to make contact, swirled through my mind.

My chief anxiety was not the logistical one of meeting her in Rome but the loyalty I felt I owed my mother and her youngest brother, Prosper.

Caitlin's scandalous conduct was generally forgiven by my grandmother but unequivocally not so by my mother and Prosper, both of whom expressed an unfettered loathing for their aunt.

I was quite prepared for Caitlin to be foul to me. From reports of her conduct I expected little else.

But what if, when I met her, I found her to be sympathetic? What if we struck up a warm relationship, just as Nicolette was hoping? What if we became friends? Would my mother and uncle feel betrayed?

That first evening on Ischia, Hilary and I decided we should explore the island. After driving round, we stopped at a small town called Forio, parked the car and wandered down the main street.

Ignoring the blandishments of various welcoming restaurants, I decided to swing through an arch and descended some steps down to a small harbour.

Yet more neon-lit restaurants beckoned us but the one that caught my eye was on a jetty, as I remember. It had no neon lights. Stray cats ambled idly across the outside tables. It had an irresistible run-down magic.

We settled at a table inside. Our sole companion was an elderly gentleman wearing a dark beret. We watched him slowly counting a wad of lire into a wicker basket.

I then found myself gazing at the pictures around us. I remarked to Hilary how much I liked them.

The man with the beret looked up, then expressed his appreciation. He had painted the pictures. His name was Felippe. He was the owner of the restaurant which he ran with his wife.

We fell into conversation. Felippe told us how he came over to Ischia during the war as an American GI and decided to settle here afterwards. He had married a local girl and created this, the first restaurant on the island.

'All the great artists and writers came here,' he declared. He trotted off some impressive names, including Truman Capote and Tennessee Williams. 'The writer I most admire of that period,' I declared, 'is W. H. Auden.'

'I knew him well,' Felippe retorted. 'I was Best Man at his wedding.'

I dimly remembered that although W. H. Auden had indeed been married, it was known as a 'lavender marriage' to protect Erika Mann, daughter of the writer Thomas Mann from persecution in Nazi Germany.

Pertinently, this unconsummated marriage was before the war, not after it. The dates did not match.

So was my genial host somewhat of a fantasy merchant, a Baron von Munchausen?

'All this talk of artists reminds me to make contact with my Great Aunt Caitlin,' I murmured obliquely.

Felippe peered at me closely.

'Are you talking of Caitlin Thomas?' he inquired.

Marvelling at his having guessed who my aunt was, I looked at the man with renewed interest.

'Yes.' Felippe peered down at his glass. 'She lived here, you know,' he said.

'In Ischia?' I replied with incredulity. 'Not just Ischia, but here,' he said waving his hands.

'I mean here in this house. After Dylan died, she moved here with her son Colm. Caitlin took a room right above the table where you are sitting. It was here she wrote her book, Leftover Life To Kill.'

Leftover Life To Kill is the famous account of her life with Dylan. I became dizzy with disbelief. This could not possibly be true.

'Are you really her nephew?' he asked. Are you Baron von Munchausen? I nearly replied.

But instead, I simply nodded and watched as the old man slowly climbed upstairs. Minutes later, he returned clutching a thick bundle of faded A4 paper. 'This is her first draft. She left it behind. You ought to have it.

'Some time soon I am going to die. It shouldn't be lost.'

He plonked the typed manuscript, full of corrections in coloured ballpoint, on the table. Shock and surprise were words that are barely adequate to describe quite what I then felt. It was like winning the lottery without the bother of buying the ticket. What strange force of destiny had led me to this place?

That night I sat up and read the first draft of Leftover Life To Kill from cover to cover, finishing at dawn.

Everything that had repelled my mother and uncle about Caitlin became clearer. Even with the privilege of self-censorship, Caitlin emerged from the pages as an unfeeling bully. I scrunched up my grandmother's note. I never called Caitlin.

The following evening I had arranged to pick up Felippe from the restaurant at six o'clock.

I was to show him our William Walton villa, about which he was most curious. I arrived 20 minutes late. While waiting he had slipped on the pavement and had to be taken to hospital with a broken hip. I later went to see him while he was in recovery.

'It is a good thing that you have the manuscript,' he said on his sickbed.

'If it is still here when I die, my wife will burn it. She is not a woman who understands such things.'

The holiday finished, I returned to London, clutching the first draft of Caitlin's book. I had not quite fulfilled my grandmother's wishes but, in a sense, I had come as close to Caitlin as I could have wished.

Some months later I was offered £1,000 for the manuscript by the University of Wales. I was then an itinerant, renting rooms in London flats. What protection could I offer such a document? It could easily be lost or stolen.

None of my immediate family wanted it. It seemed a shame to pass up such an opportunity. So I succumbed.

Days after I sold the manuscript, I rang Felippe in Ischia to tell him the news. I wanted him to share the spoils. He never came to the phone. Later I learned he had recently passed away.

I was chided by friends for not selling the manuscript to one of the American universities, any of which would have paid many thousands more. But it seemed then - and still does - a fitting and patriotic finale to the episode that to this day it lies in Wales where Dylan and Caitlin played out so much of their extraordinary life.

I have since asked a statistician the chances of someone stumbling by accident on his great aunt's home in a foreign country about which he knew nothing following the death of her husband some two years before he was born. Think of some billions to one; it is almost incalculable.

Some years after the Italy trip, I had a brief encounter with Caitlin at a family drinks party in Chelsea. I felt uneasy about selling her manuscript but never got the chance to mention it.

When my mother introduced me to her aunt, Caitlin scanned me up and down like a cold bird of prey and then moved on to someone else. Dressed in an expensive tweed outfit, she was by that point off alcohol, her years of abuse and excess behind her.

On July 31, 1994, Caitlin died. At her funeral, an event which was covered by national media, our side of the family was unrepresented.

Why did I not go? Our relationship was remote but even so I had planned to attend until a relative reminded me of the pain she so liberally spread. My absence at her funeral could, I suppose, be interpreted as a gesture of solidarity with those she had hurt so badly so many times for so many years.

But to this day I wonder if I did the right thing.

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Caitlin punched my grandmother to the ground ...she wanted to get at the drinks